Osteoporosis affects more than 75 million women and men yearly, world-wide, at annual costs > $50 billion. To define compositional changes in osteoporotic bone, we pioneered Fourier transform infrared spectroscopic imaging (FTIRI) for measurement of bone biopsies at a spatial resolution of ~7um and applied this technique along with histology, micro-CT and mechanics to evaluate therapies for osteoporosis. Bisphosphonates (BP), to date, are the most effective drugs used to prevent fragility fractures and combat bone loss. These drugs can have a half-life in bone >10 years due to their ability to bind to bone mineral. Recently, reported adverse events associated with the use of BPs triggered the notion of a drug holiday. During this holiday, BP treatment is ceased after variable periods of usage, followed by arbitrary BP-free periods. There is no definitive statement on if, or when this drug holiday should start or how long it should last. Moreover, the effect ofa drug holiday on bone quality is unknown. Bone Quality defines those factors including composition, micro-architecture and presence of microcracks, which along with Bone Quantity contribute to fracture risk. Our studies showed BP treatment decreased bone material heterogeneity at the micron-level. This loss can contribute to crack propagation and hence fracture. Our underlying hypothesis is that loss of heterogeneity, evidenced through FTIRI and micro-CT, contributes to fracture risk in BP-treated patients. We further hypothesize that Drug Holidays alone, do not alter bone quality as determined by changes in composition or heterogeneity. Based on these ideas, and the preliminary finding that PTH increases heterogeneity, we suggest a treatment approach using a BP followed by PTH. Little information exists on the effects of such treatments on bone composition at the micro- or nano-levels, yet this information is important to enable investigators to design clinical trials and recommend therapies. We will address our hypotheses using 2 specific aims.1) To characterize the composition of bone before and after a BP holiday, using iliac crest biopsies from: a) Patients after variable periods of BP-treatment followed by a 2 yr. holiday or PTH-treatment. b) Patients on long-term BP with variable holiday periods. c) Patients on long-term BP with high and normal turnover. d) Parallel studies in mice to determine at what point in time BPs attain their peak effect on tissue heterogeneity and whether or not heterogeneity changes with time. Microcracks will also be measured in iliac crest biopsies. 2) To test the hypothesis that bone heterogeneity extends to the nano-level, and that alterations in both micro- and nano-heterogeneity impact bone mechanical properties. We will i) Develop and apply nano- IR to define mineral and matrix heterogeneity in healthy, diseased and treated osteonal and cancellous animal bones. ii) Confirm our findings by analysis of human PTH-treated tissues and their controls. iii) Correlate tissue heterogeneity (from FTIRI and nano-IR) with mechanical performance at the macro- and nano-levels. If successful, these studies will address current debates regarding treatment protocols for osteoporosis.